Hey! I hope you are doing well. When you sell commission for commercial purposes, you define who owns the rights to the artwork and how it can be used. In your case, you want the rights to your work, but you are going to give the client a license to use it for commercial purposes. For example: 
Also, you can add something like this in your clause:
YOU ARE ALLOWED TO
• Create Unlimited Physical End Products*
• Create Unlimited Digital End Products*
• Create your own items that you sell on Print-on-Demand (POD) sites*
YOU ARE NOT ALLOWED TO
• Resell, sub-license, share or (re)distribute any of the digital files.
• Files cannot be transferred to 3rd parties (e.g. clients, partners, customers). In case the 3rd party needs to receive the actual font file, a separate license needs to be purchased.
I used to work in a company as a professional administrative assistant, specifically working directly to create contracts and subcontracts. If you want to make a more professional contract, you can do something like this.
ARTWORK LICENSE AGREEMENT
This Agreement is made and entered into on this _______ day of _________, 2023, by and between _________________, Artist, and _________________, Client.
1. ARTWORK DESCRIPTION
A detailed description of the artwork
2. SCOPE OF LICENSE
Specific details about where, when, and how the client is permitted to use the artwork.
3. FEE AND PAYMENT
Specific details about the amount, payment schedule, and method of payment
4. COPYRIGHT
The Artist retains copyright in the Artwork and merely licenses its use as specified in this Agreement.
5. TERMINATION
Specific conditions under which the contract may be terminated.
6. WARRANTY
A statement about the originality of the work and non-infringement on other copyrights.
7. INDEMNIFICATION
A statement to protect both parties in the event of legal claims.
Artist's Signature Date
Client's Signature Date
You can create it on Google Documents for the client to sign the terms and conditions if everything is an online transaction. Also, you can send an invoice through Paypal, where you can add the terms and conditions too. I hope this help. Remember, if you can afford it, consider consulting with a legal professional.